1. Field of the Invention
The present invention relates to electromagnetic shielding of an electrical apparatus wherein an enclosure made of non-conductive material such as a resin accommodates a circuit board with various circuit devices.
2. Description of the Related Art
An electrical apparatus produces unexpected electromagnetic radiation (referred to as unwanted radiation in the following description). Considering environmental effects of unwanted radiation, each country has legal regulations providing an acceptable level of radiation. It is therefore desired for electrical apparatus to reduce unwanted radiation down below a prescribed level in a targeted country.
Conductive coating of an inner surface of an enclosure is known for shielding of unwanted radiation produced in an electrical apparatus so as to prevent the radiation from leaking outside. This method of shielding allows in principle the entire enclosure to be a closed electromagnetic shielding space. It is thus possible to prevent leakage of unwanted radiation whose source is in the enclosure.
However, an electrical apparatus enclosure generally has openings for displays and keypads and so on. The openings act as outlets for unwanted radiation and therapy reduce the benefit of electromagnetic shielding. A metal shielding case has been therefore used for enclosing only the unwanted radiation source.
However, the addition of shielding case requires additional fabrication and installation steps. Manufacturing costs of the apparatus as a whole thus increase.
It is an object of the invention to provide electromagnetic shielding allowing unwanted radiation from an electrical apparatus to be locally shielded.
An electromagnetic shield is provided for an electrical apparatus having a circuit board with circuit devices accommodated in an enclosure made of non-conductive material. The electromagnetic shield comprises: a first conductor pattern maintained at a fixed potential, extended on the circuit board to surround a source of unwanted electromagnetic radiation among the circuit devices; a rib structure extended over an inner surface of the enclosure and corresponding to the conductor pattern; and a conductive coating for coating at least a region of the inner surface of the enclosure from a top of the rib structure to a section surrounded by the rib structure.
The present invention may be further configured such that a second conductor pattern with a shape corresponding to the first conductor pattern is formed on another surface of the circuit board while the second conductor pattern and the first conductor pattern are electrically connected to each other with through holes; a second-rib structure with a shape corresponding to the rib structure is extended over an inner surface of the enclosure on another-side of the circuit board; and a second conductive coating coats at least a region of the inner surface of the enclosure from a top of the second rib structure to a section surrounded by the second rib structure.
The conductive coatings of the invention may be formed through coating of conductive agent, for example.
The electromagnetic shield of the invention allows the conductor pattern to be brought to absolute contact with the top of the rib structure coated with the conductive coating when the circuit board is installed in the enclosure. As a result, an electromagnetic shielding space surrounding the source of unwanted electromagnetic radiation is locally formed in the enclosure. In addition to this structure, the electromagnetic shielding of another aspect of the invention allows the second conductor pattern to be brought to absolute contact with the top of the second rib structure coated with the conductive coating when the circuit board is installed in the enclosure. As a result, an electromagnetic shield space is formed over both sides of the circuit board.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings.